This proposal takes a novel approach to the etiology of secondary injury that occurs after neurotrauma. It investigates the role of the immune system in mediating delayed neuronal and myelin degeneration precipitated by traumatic spinal cord injury (SCI). The studies proposed here will utilize a highly characterized SCI model capable of producing lesions similar to those found in the clinical setting. This project will take advantage of an established consortium with existing expertise in immunological approaches, SCI, and cytokine analysis that may provide insights into the mechanisms of trauma-mediated secondary degeneration after spinal injury. It is the major hypothesis of this study that both intrinsic CNS inflammatory cells (microglia) and peripheral blood mononuclear cells (monocytes/macrophages, lymphocytes) are involved in the initiation of cytotoxic cascades which contribute to the production of specific CNS lesions, particularly in the white matter, for long time periods after SCI. Our approaches, using the Lewis rat strain, will allow us to: (1) define the nature and activation of cellular immune elements that may be involved in progressive tissue necrosis (immunocytochemistry/lectin histochemistry), (2) define the role of peripheral macrophages vs. activated parenchymal microglia in the secondary pathology of spinal cord trauma by selectively depleting peripheral macrophage populations in parallel with immunological and physiological assays, (3) estimate the effects of these depletion protocols on behavioral paradigms designed to assess functional recovery after SCI (behavioral paradigms designed for in the Ohio State Spinal Cord Injury Research Center), (4) correlate the presence of inflammatory infiltrates with the expression of various cytokines known to be involved in the injury process (measure mRNA by Northern blotting/quantitative RT-PCR before and after macrophage depletion), and (5) assess changes in spinal cord blood- brain barrier (BBB) integrity, patterns of lymphatic drainage, and peripheral lymphoid sensitization after SCI ([14C]-labeled alpha- aminoisobutyric acid (AIB), [14C]-labeled polyethylene glycol, and proliferation assays/adoptive transfer techniques respectively). In this manner, major questions about the role of the immune system in delayed traumatic injury may be answered and appropriate therapeutic regimens designed.